1. Field of the Invention
The present invention relates to a repair of a pattern defect of a base that is obtained by a technique of screen printing, e.g., plasma display panels. More particularly, the invention relates to a repair of a pattern defect of a printed material layer, which is performed after forming a variety of material layers, such as a dielectric layer and an electrode layer, on a substrate by a technique of screen printing.
2. Description of the Background Art
Screen printing (which is also called thick film printing) is a printing method in which a screen plate is placed in close proximity to a base and a pasty printing material is transferred from a screen mesh of a desired pattern to the base. This printing method is employed in forming a pattern of an electrode wire layer, a dielectric layer, a barrier rib portion, a phosphor layer or the like, on a substrate constituting a plasma display panel (PDP). Alternatively, this method is employed in forming a phosphor layer of a CRT (cathode ray tube), an electrode layer, a dielectric layer or the like of a printer head substrate.
In screen printing, after a printing material paste is transferred onto a substrate, a firing or heating process is performed under conditions suitable for the printing material paste to evaporate unneeded ingredients in the paste, thereby forming an electrode wire layer, a dielectric layer and the like on the substrate.
Such a screen printing is favorable for reducing manufacturing cost because of its low device cost and running cost. However, its pattern transfer accuracy is not so high and it is susceptible to a pattern defect. Various improvements have been achieved to improve the transfer accuracy. However, for example, when a pattern of an electrode wire of a PDP is formed on a substrate (i.e., a base 35) by printing as shown in FIG. 7, defects might occur at several points in the overall electrode wire of the substrate. For the panel of a display device, such as a PDP, a printed pattern defect directly deteriorates the display quality of the display device. Therefore, if a printed defect occurs, it is necessary to repair the defect.
In conventional repair of printed pattern defects, a pattern defect is found by observation with a microscope or a naked eye, and then the repair material paste 37 having the same composition as the printing material of a material layer is printed to a defect position by using a repair screen 36 as a screen plate for repair, as shown in FIG. 8. A repair pattern 38 having a width identical to the pattern width of a material layer repaired, as shown in FIG. 9, has been formed in the repair screen 36. A repair printing is performed by squeezing the repair material paste 37 from the repair pattern 38 to a defective portion by using a squeegee 39. Thereafter, in order to fire the repair material paste 37 of the defective portion, the substrate is put into a firing furnace as shown in FIG. 10, having a construction similar to that in firing printed patterns, and then subjected to a re-firing for the entire surface of the substrate.
The conventional pattern defect repairs by means of printing, however, have difficulties in alignment between the repair screen 36 and a pattern defect. Specifically, a screen printing causes a mismatch of patterns between a screen pattern and a pattern of paste that has actually been printed on a base. Therefore, such a mismatch must be taken into consideration when making an alignment between a screen plate and a pattern defect. Also, a pattern to be printed may actually vary with the kind of screen pattern and printing paste used and situations where a printing is performed. Thus, the pattern defect repair by printing requires much time to adjust an alignment, resulting in a poor working efficiency.
In addition, since a repair screen 36 on which a material paste for repair has been put is placed on a substrate, the surface of the substrate might become dirty.
Moreover, due to the characteristics of screen printing, it is difficult to transfer a material paste for repair into a local defective portion so as to have a uniform thickness, and a variation is liable to occur. Further, a repaired portion tends to upheave, which might adversely affect the smoothness and the like of other material layers that are formed thereon after repair.